1. Field of the Invention
This invention relates to the control of electromagnetic actuators in photographic apparatus, and more particularly to the control of actuators of the type having an armature mounted for movement in a magnetic field and means connected to the armature for urging the armature toward a rest position, and being particularly adapted for control by a digital computer.
2. Discussion Related to the Problem
The trend in photographic camera design is toward the use of a small digital computer called a microcomputer, to control all camera functions. The microcomputer accepts digital inputs from a viarety of transducers in the camera, such as scene light measuring apparatus, camera mechanism position indicators, automatic range finders, and the switches and buttons that are adjusted by the photographer. The microcomputer responds to these inputs to produce control signals for various functions in the camera, such as lens focus, aperture size, shutter operation, mirror movement, and film advance. The control signals, as produced by the microcomputer, are in a digital format; thus necessitating the step of digital-to-analog conversion when an analog output transducer is employed. Wherever possible, it is desirable to eliminate the step of digital-to-analog conversion by employing a transducer that can be driven directly by a digital signal, thereby simplifying the camera mechanism and reducing the overall manufacturing cost. U.S. Pat. No. 4,024,552 issued May 17, 1977 to Toshihiro Kondo, discloses a relatively simple electromechanically actuated optical blade comprising a rectangular planar conductive coil embedded in an opaque plastic blade. The blade is slidably mounted in grooves in the camera body and one of the legs of the rectangular coil resides in the vicinity of a magnetic field that is directed perpendicularly to the plane of the coil. When a current flows in the coil, a force generally perpendicular to one leg of the rectangular coil, is generated; causing the blade to slide in the grooves. Return springs are provided to return the blade to its starting position when the current ceases. Flexible extension leads supply current to the coil from a source within the camera. In one disclosed embodiment, a pair of blades having triangular shaped cutouts cooperate to form progressively larger apertures as the blades move away from each other. The pair of blades are employed as a combination shutter and aperture; and the aperture size is determined by interposing adjustable stops in the paths of the blades.
An improved electromechanical actuator of the planar coil variety is disclosed in U.S. patent application Ser. No. 219,168, filed Dec. 22, 1980 in the name of J. K. Lee and assigned to the present assignee. The actuator, shown in FIG. 1, includes an armature generally designated 20, having a planar coil 22 carried by a coil support member 24. Coil support member 24 is preferably formed from a sheet of insulating material. The coil support member 24 is mounted for movement by a .lambda.-type flexure hinge 26 that defines a virtual pivot point 28 at a point where the extension of the legs of the .lambda. hinge intersect. The flexure hinge provides a low friction mounting and an automatic restoring force to return the armature to an initial position after each operation. Planar coil 22 defines first and second legs 30 and 32 generally perpendicular to the direction of movement of the coil above pivot point 28. A first magnetic field B.sub.1, generally perpendicular to the plane of the coil 22 and indicated by the arrowheads as pointing out of the drawing in FIG. 1, lies in the vicinity of the first leg 30. A second magnetic field B.sub.2, generally antiparallel to the first magnetic field and indicated by arrowtails as pointing into the plane of the coil in FIG. 1, lies in the vicinity of the second leg 32. Planar coil 22 is electrically connected to a control circuit 34. Current for coil 22, supplied by control circuit 34, is carried along the legs of flexure hinge 26. An aperture blade 40 carried on an end of the coil support member opposite from coil 22, forms a tapered aperture 42 for progressively uncovering a fixed aperture 44 in a camera mechanism plate 46.
Referring now to FIG. 2, when control circuit 34 causes a current i.sub.1 to flow in leg 30 of planar coil 22, and a currrent i.sub.2 of equal magnitude, to flow in the opposite direction in leg 32 of coil 22, a force F.sub.1 is generated on the first leg 30 of planar coil 22 and a force F.sub.2 is generated on the second leg 30. The forces F.sub.1 and F.sub.2 act together to rotate armature 20 around virtual pivot point 28 in the direction of arrow A to displace the aperture blade 40 with respect to the fixed aperture 44, thereby uncovering the fixed aperture as shown in FIG. 2. The amount of movement of aperture blade 40 is controlled by controlling the amount of current in coil 22. When current ceases to flow in planar coil 22, flexure hinge 26 returns the actuator to its original position as shown in FIG. 1.
If position control is attempted by simply controlling the amount of current supplied to the actuator, static friction perturbs the final position achieved by the armature, resulting in position error. The effects of static friction are overcome, and a more reliable, repeatable position control is achieved by applying the driving current in pulses. This technique provides a small dither signal that breaks the remaining static friction. The frequency of the pulses in the control signal is chosen to be somewhat above the cutoff frequency on the frequency versus response curve of the actuator. In a preferred embodiment, the pulse duration is modulated to control the aperture size, and the number of pulses applied to the actuator controls the shutter time.
FIG. 3 illustrates a pulse train of the type used to control the actuator. The pulse train comprises a series of 50 percent duty cycle "leader" pulses, centered about 0 Volts, that shake the actuator to break the static friction, without actually causing any substantial movement of the actuator. Next come a series of positive "open" pulses that cause the actuator to move to a position determined by the duty cycle of the open pulses. Finally, a series of 50 percent duty cycle "trailer" pulses allow the actuator to return to the starting position while breaking the static friction. The dashed line labeled 100 in FIG. 4, illustrates the resulting shutter profile in terms of percentage aperture opening with respect to time. As shown in FIG. 4, the shutter was programmed to close as soon as it reached approximately 100 percent aperture opening. The resulting shutter time (t.sub.1) represents the shortest shutter time achievable at full aperture.
As can be seen from FIG. 4, the resulting shutter profile is rather "inefficient" in the sense that most of the exposure occurs at apertures less than the desired maximum aperture. Furthermore, in order to provide a more versatile actuator, it is desirable to have shorter shutter times at maximum aperture. For example, experience has shown that typical examples of shutters of the type shown in FIG. 1, are capable of achieving shutter speeds of approximately 1/250 of a second at maximum aperture, shutter speeds of 1/500 to 1/2000 of a second would be desirable. It has not proven to be practical to obtain high shutter speeds by increasing the shutter power and spring stiffness. Doubling the shutter speed would require four times the current and four times stiffer springs, resulting in the use of sixteen times more power to operate the actuator. Furthermore, it has been found that at small apertures, the complex nature of the friction in the shutter results in significant variations in behavior from one shutter actuator to another, and for very small apertures, some shutters fail to open at all. Another problem noted with the planar electromagnetic shutter actuator was the tendency for the shutter to "bounce open" after returning from large apertures.
The problem faced by the inventor therefore, was to increase the speed and reilability of the electromagnetic actuator without substantially increasing the amount of power consumed by the actuator.